Automatic Forearm twist?
I found the solution I was looking for. The trick is to not use the same bones for twisting and IK. Use one set of bones for twisting, and another set for IK.


Setup a rig similar to what I intially mentioned (two forearm bones for twisting)
Add a new "parent forearm" bone that will serve for IK (or manual posing) purposes, parented by the upper arm or shoulder bone. It will run from the elbow, all the way to the hand. Set this bone to not deform. For IK, this bone serves as the primary forearm. For FK, it serves as a handle, basically. A container - so you don't have to manually pose both twist bones. If you export for game engines, you won't need to include this bone.
Setup both twist forearm bones as children to the parent forearm. These should be full deforming bones with weights or whatever.
Set the hand bone as a child of the parent forearm, inheriting full rotations.
Add the IK constraint to the parent forearm bone. You will need some other bone as the target for the chain.
Add a copy rotation of local Y to the twist forearm bone closest to the hand. If you use more than one twist bone, you will need to add this to the others as well.


I think that's it. Let me know if I missed something. I found this video, which helped tremendously: https://www.youtube.com/watch?v=Vm7ST03SW8Y ..I honestly don't like the way he set up his vertex weights, and don't think its necessary to use more than one joint for forearm twisting, but otherwise, the setup seems solid and achieved exactly what I needed. Hope this helps someone looking to do the same thing.